Predators can be classified by their consumer-resource interactions[2] with their prey. Two factors are considered here: how close the predator and prey (or host) are, and whether the prey is directly killed by the predator, where true predation and parasitoidism involve certain death.

Grazing animals generally do not kill their prey, but like predators, they live by feeding on other organisms. While some herbivores like zooplankton live on unicellular phytoplankton and therefore inevitably kill what they eat, in a relationship sometimes called predation,[14] many others including cattle and sheep only eat a part of the plants that they graze.[15] Many species of plant are adapted to regrow after grazing damage. For example, the growing meristems of grasses are not at the tips as they are in most flowering plants, but at the base of the leaves.[16] Similarly, kelp is grazed in subtidal kelp forests, but continuously regrows from a meristem at the base of the blade where it joins the stipe.[17] Animals may also be 'grazed' upon; micropredators[18] such as female mosquitoes land on hosts briefly to feed on blood. Herbivore-plant interactions, as with predator-prey interactions, have driven plants to evolve defences such as thorns and chemicals to dissuade grazing.[4]

Micropredators interact with their hosts much like parasites, not killing their hosts. Parasitoids sooner or later kill their hosts, while predators kill their prey immediately. There is a continuum between these feeding strategies.

Parasites, like predators, live by feeding on another organism. The entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one".[19] Parasites often do not kill their hosts, the exception being parasitoids which always do, thus blurring the line between parasitism and predation.[4] Equally, small parasites such as mosquitoes exploit their hosts much as micropredators such as moth caterpillars on an oak tree and grazers do, though endoparasites in particular have a close association with their host species; again there is essentially a continuum between these feeding interactions.[2][20]

Parasitoids are organisms living in or on their host and feeding directly upon it, eventually leading to its death. They are much like parasites in their close associations with their hosts. Unlike typical parasites, they always kill their hosts, but often not instantly. Parasitoid wasps are solitary insects that live a free life as adults, laying eggs on or in other insects such as lepidopteran caterpillars. The wasp larvae feed on the growing host, eventually killing it. Parasitoids make up as much as 10% of all insect species.[21][22]

Predators are often highly specialized, where for example the Eurasian lynx only hunts small ungulates.[23] Others such as leopards are more opportunistic generalists.[24] The specialists may be highly adapted to capturing their preferred prey, whereas generalists may be better able to switch to other prey when a preferred target is scarce. When prey have a clumped (uneven) distribution, the optimal strategy for the predator is to be more specialized as the prey are more conspicuous and can be found more quickly.[25]

Predators are often another organism's prey, and likewise prey are often predators. Though blue jays prey on insects, they may in turn be prey for cats and snakes, and snakes may be the prey of hawks. One way of classifying predators is by trophic level. Carnivores that feed on heterotrophs are secondary consumers; their predators are tertiary consumers, and so forth. Because only a fraction of energy is passed on to the next level, this hierarchy of predation must end somewhere, and very seldom goes higher than five or six levels. For example, a lion, an apex predator (at the top of its food chain) that preys upon large herbivores such as wildebeest, which in turn eat grasses, is only a secondary consumer. Other apex predators include the sperm whale, Komodo dragon, tiger, and most eagles and owls. Many predators eat from multiple levels of the food chain. A carnivore may eat both secondary and tertiary consumers, and its prey may itself be difficult to classify for similar reasons.[26]

An alternative view offered by Richard Dawkins is of predation as a form of competition: the genes of both the predator and prey are competing for the body (or 'survival machine') of the prey organism.[27] This is best understood in the context of the gene centered view of evolution. Another manner in which predation and competition are connected is throughout intraguild predation. Intraguild predators are those that kill and eat other predators of different species at the same trophic level, and thus that are potential competitors.[28]

Predators may increase the biodiversity of communities by preventing a single species from becoming dominant. Such predators are known as keystone species and may have a profound influence on the balance of organisms in a particular ecosystem.[29] Introduction or removal of this predator, or changes in its population density, can have drastic cascading effects on the equilibrium of many other populations in the ecosystem. For example, grazers of a grassland may prevent a single dominant species from taking over.[30]

The elimination of wolves from Yellowstone National Park had profound impacts on the trophic pyramid. Without predation, herbivores began to over-graze many woody browse species, affecting the area's plant populations. In addition, wolves often kept animals from grazing in riparian areas, which protected beavers from having their food sources encroached upon. The removal of wolves had a direct effect on beaver populations, as their habitat became territory for grazing.[31] Furthermore, predation keeps hydrological features such as creeks and streams in normal working order. Increased browsing on willows and conifers along Blacktail Creek due to a lack of predation caused channel incision because the beavers helped slow the water down, allowing soil to stay in place.[31]

In social predation, a group of predators cooperates to kill creatures larger than those they could overpower singly. Social predators such as ions, hyenas, and wolves collaborate to catch and kill large herbivores. By hunting socially chimpanzees can catch colobus monkeys that would readily escape an individual hunter, while a cooperating group of Harris hawks can trap rabbits.[33][34]

In size-selective predation, predators select prey of a certain size. Large prey may prove troublesome for a predator, while small prey might prove hard to find and in any case provide less of a reward. This has led to a correlation between the size of predators and their prey.[35] Size may also act as a refuge for large prey. For example, adult elephants are relatively safe from predation by lions, but juveniles are vulnerable.[35]

Many antipredator adaptations have evolved in prey populations due to the selective pressures of predation over long periods of time. Some species mob predators cooperatively. Others such as Thomson's gazellestot to signal to predators such as cheetahs that they will have an unprofitable chase.[36] Many prey animals are aposematically colored or patterned as a warning to predators that they are distasteful or able to defend themselves.[37] Such distastefulness or toxicity is brought about by chemical defenses, found in a wide range of prey, especially insects, but the skunk is a dramatic mammalian example.[38] Chemical defences can include toxins, such as bitter compounds in leaves, often absorbed by leaf-eating insects such as caterpillars and used to dissuade potential predators.[39]

Camouflage of the dead leaf mantis makes it less visible to both its predators and its prey.

Predators use stealth, camouflage and aggressive mimicry to improve their hunting efficiency. Members of the cat family such as the snow leopard (treeless highlands), tiger (grassy plains, reed swamps), ocelot (forest), fishing cat (waterside thickets), and lion (open plains) have coloration and patterns suiting their habitats.[43] Female Photurisfireflies, for example, copy the light signals of other species, thereby attracting male fireflies, which they capture and eat.[44]Flower mantises are ambush predators; camouflaged as flowers, such as orchids, they attract prey and seize it when it is close enough.[45]Frogfishes are extremely well camouflaged, and actively lure their prey to approach using an esca, a bait on the end of a rod-like appendage on the head, which they wave gently to mimic a small animal, gulping the prey in an extremely rapid movement when it is within range.[46]

Predators tend to lower the survival and fecundity of their prey, and depend on prey for their survival, so predator populations are affected by changes in prey populations and vice versa.[47] The population dynamics of predator–prey interactions can be modelled using the Lotka–Volterra equations. These provide a mathematical model for the cycling of predator and prey populations. Predators tend to select young, weak, and ill individuals.[48]

Predation appears to have become a major selection pressure shortly before the Cambrian period—around 550 million years ago—as evidenced by the almost simultaneous development of calcification in animals and algae,[49] and predation-avoiding burrowing. However, predators had been grazing on micro-organisms since at least 1,000 million years ago.[50][50][51][52][53]

In biological pest control, predators from a pest's natural range are introduced to control populations, at the risk of causing unforeseen problems. Natural predators are an environmentally friendly and sustainable way of reducing damage to crops, and are an alternative to the use of chemical agents such as pesticides.[58]

^Cramp, Stanley, ed. (1977). Handbook of the Birds of Europe, the Middle East and North Africa, the Birds of the Western Palearctic. 1: Ostrich to Ducks. Oxford University Press. p. 332. ISBN0-19-857358-8.

^Wilson, Edward O. (2014). The Meaning of Human Existence. W. W. Norton & Company. p. 112. ISBN978-0-87140-480-0. Parasites, in a phrase, are predators that eat prey in units of less than one. Tolerable parasites are those that have evolved to ensure their own survival and reproduction but at the same time with minimum pain and cost to the host.

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